In case Java enum has an abstract member, it has the ACC_ABSTRACT flag
set in the bytecode. However, we should still load it with final
modality to be consistent with Kotlin enums which are always considered
final
#KT-23426 Fixed
Note that the quick fix to implement header class works incorrectly when
that class has nested classes at the moment; this should be fixed
separately
#KT-15494 Fixed
#KT-18573 Fixed